The present invention relates to a curable composition which has adhesiveness or stickiness, namely an adhesive curable composition. More particularly, it relates to a sealing composition which shows good weathering resistance and heat resistance, has good handling properties owing to its low viscosity, can be packed in one container and shows good coating properties, and to a coating composition or pressure sensitive adhesive composition which has good weathering resistance and heat resistance in which the solvent content can be markedly reduced (to attain a high solid content) owing to the intrinsic low viscosity of said composition and which is therefore less unfriendly to the environment.
In the field of building and construction, among others, silicone type sealing materials having a silicon-containing group which has a hydroxyl or hydrolyzable group bound to a silicon atom and can be crosslinked under formation of siloxane bonds (hereinafter referred to also as xe2x80x9ccrosslinkable silyl groupxe2x80x9d) have so far generally been widely used as sealing compositions excellent in weathering resistance and heat resistance. It is pointed out, however, that silicone-based sealing materials have drawbacks such as poor adhesion of paints and tendency toward staining areas around joints, although they are excellent not only in weathering resistance but also in resistance to movement and cold workability, among others.
Recently, crosslinkable silyl-terminated polyisobutylene-based sealing materials have been proposed as new-type weathering-resistant sealing materials. It is pointed out, however, that polyisobutylene-based sealing materials are high in viscosity, hence poor in workability, and hardly permeable to moisture and therefore difficult to subject to one-component packaging, although they are excellent in weathering resistance and resistance to permeation of moisture.
Vinyl or (meth)acrylic polymers are excellent in weathering resistance and therefore have the possibility of their being useful as base polymers in high weathering resistance sealing materials. In particular, crosslinkable silyl-containing (meth)acrylic polymers are very promising, since they have already been put to practical use in high weathering resistance paint compositions. However, said polymers are generally produced by copolymerizing a crosslinkable silyl-containing (meth)acrylic monomer and another vinyl monomer, so that crosslinkable silyl groups are randomly introduced into molecular chains. Therefore, difficulties arise in using said polymers in those elastic sealing materials which are required to have low modulus and high elongation characteristics. A vinyl or (meth)acrylic polymer, if it has a crosslinkable silyl group terminally to the main chain, would be expected to be utilizable as a new-type weathering-resistant sealing material.
On the other hand, acrylic pressure sensitive adhesives as well as natural rubber-based pressure sensitive adhesives are produced in large quantities, since they have well-balanced adhesive characteristics without the aid of any tackifier resin. Since, however, acrylic pressure sensitive adhesives are disadvantageously poor in cohesion owing to their molecular weight and molecular weight distribution, it is usual to subject them to crosslinking for cohesion improvement. For such crosslinking, various techniques have been developed. For example, methods have been proposed which comprise adding a crosslinking agent such as a polyisocyanate compound, epoxy compound, polybasic carboxylic acid, polyamine compound, phenolic resin, or sulfur compound, for instance; or carrying out crosslinking of crosslinkable silyl-containing acrylic polymers in the presence of a condensation catalyst. In particular, pressure sensitive adhesives comprising a crosslinkable silyl-containing acrylic polymer as the main component are advantageous in that they are cured by crosslinking via siloxane bonding and therefore are excellent in weathering resistance.
However, even those pressure sensitive adhesives in which a crosslinkable silyl-containing (meth)acrylic polymer is used have crosslinkable silyl groups randomly introduced therein, so that, when a low-molecular polymer is used to attain a reduced viscosity, the distance between crosslinking sites becomes short. In that case, the problem is that elastic properties required of pressure sensitive adhesives cannot be obtained. For providing pressure sensitive adhesives with elastic properties, a method is available which comprises using a high-molecular polymer as the above polymer and reducing the amount of the crosslinkable silyl-containing monomer to thereby increase the distance between crosslinking sites. However, the use of a high-molecular polymer as the above polymer leads to a high viscosity or solid state and, therefore, for using the resulting polymer as a pressure sensitive adhesive, it is necessary to use a solvent in fairly large amounts to reduce the viscosity. In the case of such a solvent type pressure sensitive adhesive, the solvent is evaporated after application of the pressure sensitive adhesive to a film or like substrate. For this, a lot of heat energy is required and, in addition, the solvent may cause a fire or adversely affect the human body. It is therefore required that no solvent be used or a high solid content be attained. The use of a high-molecular polymer as the above polymer thus has its limits.
To solve the above problem, it has been proposed that a (meth)acrylic copolymer which meets the relatively low molecular weight and low viscosity requirements and has a crosslinkable silyl group introduced therein terminally to the main chain be used as the base polymer in pressure sensitive adhesive compositions.
Crosslinking silyl-containing vinyl or (meth)acrylic polymers are also used as base polymers in high weathering resistance solvent or water paint compositions since, when cured by crosslinking in the presence of an appropriate condensation catalyst, they give coat films excellent in weathering resistance.
As a result of the recent increasing interest in the earth environment, the use of solvent paints which evaporate a large amounts of solvents is rather refrained but it is demanded that paint compositions have a still higher solid content. For achieving a high solid content in vinyl or (meth)acrylic paints while securing the spreadability in the coating step, it is generally necessary to reduce the viscosity and, therefore, to reduce the molecular weight of the polymer. However, when the molecular weight is reduced, a problem arises, namely the weathering resistance intrinsic in vinyl or (meth)acrylic polymers is lost.
As a method of solving this problem, a method is presumable which comprises reducing the molecular weight distribution, namely the ratio (Mw/Mn) of weight average molecular weight (Mw) to number average molecular weight (Mn), of a vinyl or (meth)acrylic polymer, as determined by gel permeation chromatography, to thereby reduce the polymer viscosity and thus attain a high solid content. However, those vinyl or (meth)acrylic polymers which are used in paint compositions are generally produced by free radical polymerization, so that only those polymers which have a wide molecular weight distribution (generally having an Mw/Mn value of not less than 2) can be obtained.
Further, the reduction in molecular weight for attaining a low viscosity results in a shortened distance between sites of crosslinking, which in return leads to formation, in the step of curing, of coat films with a very high crosslinking density. As a result, the coat films obtained are very poor in elastic properties and the problem that said films may not follow the deformation of substrates will arise. One means to solve this problem is to use a vinyl or (meth)acrylic polymer having a crosslinkable silyl group terminally to the main chain as the main component. The crosslinkable silyl-terminated main chain makes it possible to increase the distance between crosslinking sites while maintaining an adequate molecular weight and, as a result, to provide the resulting coat films with elastic properties.
As mentioned hereinabove, it is necessary, for obtaining low-viscosity sealing compositions, pressure sensitive adhesive compositions and paint compositions without decreasing physical properties, to obtain a vinyl or (meth)acrylic polymer having a crosslinkable silyl-terminated main chain and a narrow molecular weight distribution. It is not easy to produce such a polymer by the prior art technology.
As an attempt to synthesize such a polymer having a crosslinkable silyl-terminated main chain, a method is disclosed in Japanese Kokoku Publication Hei-3-14068, for example, which comprises polymerizing a (meth)acrylic monomer in the presence of a crosslinkable silyl-containing mercaptan, a crosslinkable silyl-containing disulfide and a crosslinkable silyl-containing radical polymerization initiator. Further, in Japanese Kokoku Publication Hei-04-55444, a method is disclosed which comprises polymerizing an acrylic monomer in the presence of a crosslinkable silyl-containing hydrosilane compound or a tetrahalosilane. It is difficult by these methods, however, to introduce the crosslinkable silyl group into the polymer at both termini with sureness. Thus, an insufficient gel fraction, hence insufficient curability, will result.
Further, Japanese Kokai Publication Hei-6-211922 discloses a room temperature-curable composition which comprises a crosslinkable silyl-terminated (meth)acrylic polymer obtained by synthesizing a hydroxyl-terminated acrylic polymer by using a hydroxyl-containing polysulfide in excess relative to an initiator, followed by conversion of the hydroxyl group. This synthetic method makes it possible to obtain a (meth)acrylic polymer with a relatively high percentage of terminal crosslinkable silyl groups but requires the use of a large amount of an expensive hydroxyl-containing polysulfide, which is a chain transfer agent. This is a problem from the production process viewpoint. Another problem is that the viscosity of the polymer becomes high since the molecular weight distribution becomes wide.
Accordingly, in view of the foregoing, the present invention has for its object to produce an adhesive curable composition excellent in weathering resistance and heat resistance and having a low viscosity by using, as the main component, a vinyl polymer having at least one crosslinkable silyl group.
The invention thus provides an adhesive curable composition which comprises, as the main component, a vinyl polymer having at least one crosslinkable silyl group represented by the following general formula (1):
xe2x80x94[Si(R1)2-b(Y)bO]mxe2x80x94Si(R2)3-a(Y)axe2x80x83xe2x80x83(1) 
wherein R1 and R2 are the same or different and each represents an alkyl group containing 1 to 20 carbon atoms, an aryl group containing 6 to 20 carbon atoms, an aralkyl group containing 7 to 20 carbon atoms or a triorganosiloxy group represented by the formula (Rxe2x80x2)3SiOxe2x80x94 (in which Rxe2x80x2 represents a monovalent hydrocarbon group containing 1 to 20 carbon atoms and the plural Rxe2x80x2 groups may be the same or different) and, when there are two or more R1 or R2 groups, they may be the same or different; Y represents a hydroxyl group or a hydrolyzable group and, when there are two or more Y groups, they may be the same or different; a represents 0, 1, 2 or 3; b represents 0, 1 or 2; and m represents an integer of 0 to 19; with the condition that a, b and m satisfy the relation a+mbxe2x89xa71.